1. Field of the Invention
The invention relates to a process and an apparatus for coating a metal strip on one side with a molten metal.
More particularly, the invention is concerned with hot galvanizing a steel strip on one side thereof. Specifically, the strip is cleaned and heated, and the strip is then conveyed to a melting bath of the molten metal above the level of the melting bath in a protective gas atmosphere. An obverse side of the strip is continuously brought into contact with the molten metal while being maintained above the level of the molten bath, and it is supported on its reverse or rear side in the area of contact.
2. Description of the Prior Art
For some years specialists have been endeavoring to develop suitable processes, apparatuses and devices for coating a metal strip on one or the obverse side with a molten metal. The aim of these attempts is to obtain an even or uniform and an exactly adjustable thickness of layer on only one side. In addition, a process is required to prevent the molten metal from reaching the rear or reverse side of the strip during the coating.
In a known process of the above mentioned type, the metal strip is conveyed at a close proximity over the level of the melting bath, for example, by means of a supporting and deflection roller. Due to the surface tension of the molten metal, a meniscus of the molten metal is formed between the surface of the bath and the metal strip and, this meniscus is maintained with the short distance between metal strip and bath surface. In this way, the passing strip is continuously wetted with molten metal. The setting or determination of the thickness of the layer of coating is achieved by stripping the excess metal when still liquid by means of a connected jet knife (see, German Offenlegungsschrift No. 2,712,003).
However, the required layer thicknesses are only obtained with this adjustment of layer thickness when the preceding layer applied is applied evenly over the entire width of the strip. An even coating over the entire width of the strip can only be achieved when the distance between the level of the bath and the strip does not fluctuate. This requirement is difficult to achieve with a very short set distance, e.g. 10 mm. When the distance between the strip material and the level of the bath fluctuates, e.g. due to changes in the level of the bath, partial or complete breakup of the meniscus can result, or the molten metal can flow over the edges of the strip thus causing undesired coating of the rear or reverse side of the strip. There are several causes of fluctuations in the layer of the bath: consumption of the molten metal by the coating process, refilling of the molten metal, blasting of the bath surface by the blast jet from the jet knife. All these difficulties can hardly be eliminated even with exact guidance of the strip and a regulation plant for supervision of the level of the melting bath.
In another known process, deflection rollers are used. The strip which is to be coated is deflected 90.degree. by two deflection rollers in each case and conveyed by them approximately parallel to the level of the melting bath at a considerable distance therefrom. The coating material is applied by means of a scoop roller immersed in the melting bath onto the strip in an unsupported area between the two deflection rollers. The desired thickness of layer is set by means of a squeeze-off roll running on the scoop roller (see U.S. Pat. No. 3,228,788).
It is indeed possible with this process and apparatus to coat the strip on only one side along its entire width without there being any danger of molten metal reaching the other side of the strip; however, adjustability of the thickness of layer is not exact enough with this process and apparatus in spite of the additional squeeze-off roll. Even a slight uneveness in the strip or movement of the strip radially to the scoop roller (shimmying) can result in an irregular contact between the strip and the scoop roller, and thus result in an uneven coating.